The need for high-speed transfer of digital data over communications lines and the national telephone network is well known. As the volume and expense of transferring such data increases, higher transfer speeds become even more desirable. Approximately 95% of the data presently transferred is done at 300 baud (approximately 30 characters/second). Modulator/demodulators (MODEMS) to convert digital data into signalling tones compatible with the telephone network and to convert the signalling tones back to digital data are used for transferring such data.
The primary disadvantage of conventional, relatively low-cost MODEMS is the slow speed at which data may be transferred. The limitation is more the fault of the telephone network than the MODEM since existing telephone networks have a limited bandwidth of around 2400 Hz. This relatively small bandwidth limits the speed at which data can be transferred by common frequency shift keying (FSK) and phase shift keying (PSK) techniques. More sophisticated signal processing techniques have been developed which allow data to be transferred at higher rates. However, MODEMS capable of processing the data in this sophisticated manner are relatively expensive and highly complex.
Another problem associated with data communication systems stems from the need to synchronize coding of the data at the sending station with decoding of the resulting tones at the receiving station. This is usually accomplished by sending a clock signal along with the data. Alternately, in asynchronous data communications, the digital message is precluded and followed by recognizable start and stop bits. Regardless of whether synchronous or asynchronous communication techniques are used, existing MODEMS are usually unable to operate properly if the frequency at which the data is transmitted is not accurately controlled.
Another problem associated with data communications systems arises from interfacing a large number of data terminals to a common data link. Each of the data terminals is capable of receiving and transmitting data at any time, and some system must be used to prevent two or more of them from transmitting on the data link at the same time. In the past, elaborate protocol systems have been used to accomplish this function.